Mobile communication devices such as cellular phones and car navigation systems are widely used. Each of those devices has a specific frequency and a specific polarization mode. For example, the GPS (Global Positioning System) such as a car navigation system employs circularly polarized radio waves. By way of example, PTL 1 has described an antenna which has a small size but has favorable characteristics of circularly polarized waves.
In recent years, the mobile communication devices have more and more functions. For example, cellular phones having the GPS function are becoming popular. Such mobile communication devices having numerous functions need to transmit and receive a plurality of radio waves having different frequencies or polarized differently. To satisfy the requirement, a plurality of antennas are mounted to individually support the types of radio waves to be used.
Each of PTL 2 and PTL 3 has proposed a shared antenna which can transmit and receive a plurality of radio waves having different frequencies and polarized differently.